The present invention relates to a form of the drug thymidine having an increased solubility.
Thymidine, whose systematic name is 1-(2-deoxy-.beta.D-ribofuranosyl)-5-methyluracil, is a constituent of deoxyribonucleic acid and may be isolated from thymonucleic acid. It has the structure: ##STR1##
Thymidine may be used to reverse, at least partially, the systemic effects of idoxuridine. Thymidine is also used as a bioassay and metabolic reagent, as a tagged carrier of radioactive tritium. Recently interest in the pharmaceutical uses of thymidine has increased, principally as an anti-cancer agent. However, investigation and use of thymidine have been hampered by its relatively low solubility in water. Although thymidine is reported in the literature to be water-soluble, this solubility is too low for many practical purposes and hampers its use. For example, in order adequately to investigate the anti-cancer activity of thymidine in aqueous solution, it would be necessary to inject some 20 liters of the aqueous solution into the patient, a quantity which is clearly unacceptable. Use of thymidine for other purposes is also enhanced by increasing its solubility, for example by requiring a smaller volume of solution.
We have now discovered a method of enhancing the water-solubility of thymidine. Thus, in its broadest aspect, the present invention provides a water-soluble form of thymidine and solutions thereof.
Surprisingly, we have found that the presence of a solubilizing agent significantly enhances the water-solubility of thymidine. Thus, the invention further consists in a composition of thymidine and a solubilizing agent and solutions of the same.
We have found that suitable solubilizing agents are salicylic acid and derivatives thereof. Preferred derivatives are amino- and methyleneamino-substituted salicylic acids. Since the administration of acidic substances for therapeutic purposes is not normally desired, it is preferred to employ the salicylic acid or derivative thereof in the form of a salt or ester. The salt is preferably an alkali metal salt, most preferably the sodium salt.
Although, as will be demonstrated hereafter, a wide range of salicylic acid derivatives has been found to be effective in the solubilization of thymidine, we prefer to employ p-methyleneaminosalicylic acid or a salt or ester thereof, in view both of their excellent solubilizing effect and their lack of side reactions in the human body. Thus, although the solubilizing effect of sodium p-aminosalicylate is as good as, or possibly better than, the solubilizing effect of sodium p-methyleneaminosalicylate, it has been suggested that sodium p-aminosalicylate may have untoward side effects, whereas sodium p-methyleneaminosalicylate has been proven safe and is thus better for general therapeutic use. On the other hand, of course, in the case of a critical cancer patient, minor side effects brought upon by sodium p-aminosalicylate may be of little practical importance.
The manner in which the two ingredients of the composition are combined may have some effect on the activity of the resulting composition and, surprisingly, I have found that the best results are achieved simply by grinding together the powders of thymidine and the solubilizing agent, especially the salicylic acid derivative. It is possible to react the thymidine and the solubilizing agent together in a suitable common solvent (e.g. methanol) and then to evaporate off the solvent to leave a solid residue. This solid residue has good water solubility but, despite carrying out this reaction under a number of different conditions, I have found that the reaction product has a therapeutic activity rather less than that of the parent thymidine, for some purposes. However, where the identical chemical activity of thymidine is not required, such a product may be used. By simply grinding together the two components, the whole of the original chemical activity of the thymidine is retained, and water solubility is increased.
It is also possible, of course, to dissolve the two ingredients of the composition (the thymidine and the solubilizing agent) in the aqueous medium in which they are to be administered to the patient and, in such a case, it is believed that the full activity of the thymidine is retained. However, for commercial purposes, it is not desirable that the ingredients should be dissolved in the aqueous medium too far in advance of administration, nor is it desirable that the separate ingredients should be supplied to the practitioner, since this could give rise to dosage errors. For this reason, for commercial purposes, the best form of the composition of the invention is that produced by grinding together the two dry powders.
Because of the way in which the composition of the invention is prepared, the exact form of liaison or interaction between the two components of the composition is unclear. It seems unlikely in the first instance, at least, that the mere grinding together of the two powders would provide any form of chemical bonding and it is, therefore, possible that the composition is a simple mixture of the two components. This concept may be supported by the fact that bringing the two components together under conditions where they would be expected to undergo some form of chemical reaction (i.e. in the presence of a good reaction solvent, such as methanol) leads to a loss of therapeutic activity, possibly as the result of freezing the structure of thymidine (which is normally believed to exist in the form of a number of tautomers) into an inactive or less active form, e.g. as follows: ##STR2##
Moreover, throughout all of the experiments described in the following Examples, the starting materials could be identified unchanged on silica gel thin layer chromatography plates eluted with 2-propanol, thus suggesting that the complex, if, indeed, it is a complex, between the thymidine and the solubilizing agent is extremely labile and suggesting very strongly the absence of any covalent bonds between the two ingredients. On the other hand, the substantial increase in solubility of the composition of the invention as compared with thymidine itself is greater than might be expected by any simple physical effects.
The precise ratio between the thymidine and the solubilizing agent is not critical to the invention and the presence of any quantity of solubilizing agent will have some beneficial effect upon solubility. Moreover, at least to some extent, the solubility of the composition increases as the content of solubilizing agent increases. Thus, too low a content of solubilizing agent has negligible solubilizing effect. On the other hand, if the amount of solubilizing agent is too great, this necessitates the administration to the patient of large quantities of a material which, at best, is neutral or of no therapeutic effect. For this reason, it is good practice to minimise the quantity of solubilizing agent to an extent consistent with achieving the desired solubilizing effect. Thus, for best results in practice, I prefer to use equimolar quantities of the thymidine and the solubilizing agent and, in any case, I prefer that the quantity of solubilizing agent should not be substantially above equimolar, since the greater the proportion of this component, the lower will be the proportion of thymidine available or the activity per unit weight of the composition.